Automatic gain control and overload protection for signal receiving systems



Jan. 13, 1970 w. G. RUSSELL 3,490,046 AUTOMATIC GAIN CONTROL ANDOVERLOAD PROTECTION FOR SIGNAL RECEIVING SYSTEMS Filed April 5, 1967 1719 2O 21 MIXERAA/D F Mom of/4L DETECTOR OSCILLATOR L IER 70 a- 70 22(-/2v) A.G.C. Mar) 23 24 AMPL/F/ER i R8 C6 1: 14.6.6. SIGNAL sou/w:

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INVENTOR WILLIAM G. RUSSELL BY PA A GENT United States Patent 3,490,046AUTOMATIC GAIN CONTROL AND OVERLOAD PROTECTION FOR SIGNAL RECEIVINGSYSTEMS William G. Russell, Kitchener, Ontario, Canada, assignor toElectrohome Limited, Kitchener, Ontario, Canada Filed Apr. 5, 1967, Ser.No. 628,648 Int. Cl. H04b N16 US. Cl. 325411 7 Claims ABSTRACT OF THEDISCLOSURE The gain of a signal amplifier of a signal receiving systemis automatically controlled by a network connected between an A.G.C.signal source and the transistor of the signal ampifier. The networkincludes an A.G.C. signal amplifier including a transistor, the baseelectrode of the latter being D.C. connected to the A.G.C. signalsource, while the emitter electrode of this transistor is D.C. connectedwith the emitter electrode of the signal amplifier transistor, a D.C.power supply being connected to both the emitter electrodes via a commonresistor. A variable impedance network is connected across at least apart of the tuned input circuit of the R.F. amplifier of the system andto the A.G.C. transistor and has a high impedance when the latter isjust conducting and a low impedance when the latter is conductingheavily, so that the variable impedance network damps the tuned inputcircuit when the A.G.C. transistor is conducting heavily.

This invention relates to improvements in automatic gain control(A.G.C.) networks for signal receiving systems, for example, radioreceivers. This invention also relates to improvements in A.G.C. andoverload protection networks for signal receiving systems.

In a radio receiver having an R.F. and an LP. amplifier each including atransistor, it is common practise to apply an A.G.C. signal to the baseelectrode of one or each transistor. However, since there will be aresistor in the emitter circuit of the transistor of the amplifier, andsince there will be a voltage drop (normally about 1 volt) across thisresistor, and, further, since the baseemitter junction voltage drop of asilicon transistor is about 0.6 volt, an A.G.C. signal change in excessof 1.6 volts will be required before a satisfactory control function canbe performed by the A.G.C. network of the receiver. In other words, insuch a system, the power requirements of the A.G.C. source arerelatively high.

In accordance with one aspect of this invention, there is provided a newand improved A.G.C. system in which the A.G.C. signal is applied to theemitter electrode of the transistor of the amplifier whose gain is to becontrolled, thereby eliminating the necessity for the A.G.C. signal toovercome the emitter voltage of the transistor, as in systems where theA.G.C. signal is applied to the base electrode of the transistor. Thisis accomplished, in accordance with the invention, by the provision ofan A.G.C. signal amplifier D.C. connected between the A.G.C. signalsource and the emitter electrode of the transistor of the amplifierwhose gain is to be controlled. The A.G.C. signal amplifier includes 'atransistor, and the emitter electrode of this transistor is D.C.connected to the emitter electrode of the other transistor. A D.C. powersupply is connected via a resistor to both emitter electrodes.

In a conventional A.M. radio receiver provided with A.G.C. when anoverload condition exists, i.e., a very strong signal is being received,even though the R.F. amplifier may be completely out olf by an A.G.C.signal, it is quite common for the signal to be coupled from the tunedinput circuit of the R.F. amplifier to the tuned ice output circuitthereof. This signal is passed to the mixer stage of the receiver wheresevere overloading occurs causing distortion of the audio signal appliedto the loudspeaker of the receiver. This problem can be overcome tosome'extent by physically locating the coils in the tuned input andoutput circuits of the R.F. amplifier as far apart as possible, but thisdoes not provide a complete solution, since the overload signal stillcan be coupled into the mixer stage via the stray capacitance of wiringand of the transistor of the R.F. amplifier.

It is known that the aforementioned overload problem can be solved bythe use of a variable impedance network that shunts the tuned inputcircuit of the R.F. amplifier. This network is virtually an open circuitwhen weak and medium strength signals are being received. However,during near overload conditions, the impedance of the network dropsappreciably, damping the tuned input circuit of the R.F. amplifier, byvirtue of which the near overload signal is attenuated at the tunedinput circuit to a value less than could cause overload.

In accordance with a preferred embodiment of this invention, a variableimpedance network shunting at least a part of the tuned input circuit ofan R.F. amplifier is provided, as well as an A.G.C. system of the typehereinbefore briefly described. The variable impedance network includesa diode connected between the tuned input circuit and the collectorelectrode of the transistor of the A.G.C. signal amplifier, and aresistor connected between this collector electrode and a terminal at areference potential, e.g., ground potential. The state of conduction ofthe transistor of the A.G.C. signal amplifier determines the impedanceof the variable impedance network. More specifically, as long as thetransistor is not conducting heavily, as will be the case for weak andmedium strength signals, the variable impedance network will bevirtually an open circuit. Under near overload conditions, however, thetransistor of the A.G.C. signal amplifier will be conducting heavilycausing the diode to conduct, thereby materially reducing the impedanceof the variable impedance network.

A feature of this system is that no positive or negative (with respectto ground) potential is required to bias the diode in the variableimpedance network off under no signal or weak and medium strength signalconditions. In other words, the D.C. potential of the electrode of thediode connected to the tuned input circuit of the R.F. amplifier can beground potential. If this were not so, additional components would berequired for application of a positive or negative bias to the diode.

This invention will become more apparent from the following detaileddescription, taken in conjunction with the appended drawings, in which:

FIGURE 1 shows one form of a radio receiver embodying this invention;and

FIGURE 2 shows a part of a radio receiver in which a differentembodiment of this invention is employed.

Referring first to FIGURE 1, there is shown an R.F. amplifier includinga transistor TRl, a tuned input circuit 10 and a tuned output circuit11. Tuned input circuit 10 consists of a coil L1 connected in serieswith the antenna 12 of the receiver, and a variable capacitor C1connected in parallel with the series circuit consisting of coil L1 andantenna 12, capacitor C1 being connected between the output terminal 13of tuned input circuit 10 and a terminal 14 at a reference potential,i.e., ground potential. Output terminal 13 is capacitively coupled via acapacitor C2 to the base electrode of transistor TRl.

Tuned output circuit 11 of the R.F. amplifier consists of a coil L2tapped at 15 and a variable capacitor C3 that may be gang tuned withcapacitor C1. One terminal of coil L2 isconnected to one terminal ofcapacitor C3, while the other terminal of coil L2 is connected to theother terminal of capacitor C3. One common terminal of capacitor C3 andcoil L2, i.e., terminal 16, is grounded. The collector electrode oftransistor TR1 is connected to tap 15.

Bias for transistor TR1 is provided by resistors R1 and R2 connected inseries with each other between a negative terminal of a DC. power supply(B-) and ground, the common terminal of resistors R1 and R2 being con-1ected to the base electrode of transistor TR1. An R.F. Jy-passcapacitor C4 is connected between the emitter electrode of transistorTR1 and ground.

A coil L3, which is transformer coupled with coil L2, .s connectedbetween terminal 16 and a mixer and local ascillator 17, which may be ofa conventional type, and Jrovides a path for applying the amplified RF.signal From the RF. amplifier to the mixer. The LP. signal from themixer is applied to an IF. amplifier 18, which nay be of a conventionaltype and whose output is sup- ;lied to a detector 19, which also may beconventional n nature. The audio signal from detector 19 is amplified ya conventional audio amplifier 20 that supplies an amplified audiosignal to loudspeaker 21. The LP. sig- 1al from amplifier 18 also issupplied to an A.G.C. sigial source 22 via a coupling capacitor C5. TheA.G.C. aignal source 22 shown in FIGURE 1 is conventional in iature andrequires no detailed description. of course, )ther types of A.G.C.signal sources may be employed. [he A.G.C. signal on line 23 connectedto the output erminal 24 of A.G.C. signal source 22 is a DC. voltage hatis negative with respect to ground. The A.G.C. sig- 1al goes lessnegative, i.e., more positive, with increasng signal strength.

An A.G.C. amplifier 25 that includes a transistor TR2 s provided. Aresistor R3 is connected between the base :lectrode of transistor TR2and ground. A resistor R4 s connected between the collector electrode oftransisor TR2 and ground with an RF. by-pass capacitor C6 )eingconnected across resistor R4. The emitter electrodes if transistors TR1and TR2 are D.C. connected, and heir common terminal 26 is connected viaa resistor R5 the negative terminal of a DC. power supply (B). )utputterminal 24 of A.G.C. signal source 22 and the ass electrode oftransistor TR2 are D.C. connected.

A variable impedance network consisting of a diode D1 .nd resistor R4 inthe collector circuit of transistor TR2 s provided. The anode of diodeD1 is connected to output erminal 13 of tuned input circuit 10, whilethe cathode if diode D1 is connected to the collector electrode ofransistor TR2.

Turning now to FIGURE 2, the circuit shown therein .iifers from thecircuit of FIGURE 1 primarily in that I.G.C. amplifier 25 is connectedto transistor TR3 of .F. amplifier 18, rather than to transistor TR1 ofthe .F. amplifier. Thus, the emitter electrodes of transistors R2 andTR3 are D.C. connected with their common erminal 27 being connected viaa resistor R6 to the .egative terminal of a D0. power supply (B). Ifesired, a second A.G.C. signal may be obtained at the utput circuit ofLP. amplifier 18 and applied to RR mplifier transistor T R1.

Returning again to FIGURE 1, resistors R1 and R2 re selected to give thedesired operating current. For xample, resistors R1 and R2 may beselected so that nder no signal conditions the emitter current oftransis- )I TR1 will be 1 ma. If resistor R has a value of 1 K., 1iswill mean that under no signal conditions the voltage t terminal 26 willbe 11 volts, whereas the voltage t the base electrode of transistor TR1will be less negave than -11 volts by approximately the base-emitterlnction voltage drop of transistor TR1.

Resistor R3 together with resistors RS and R9 deteriine the bias fortransistor T R2, and these resistors are hosen so that under no signaloperating conditions, transistor TR2 will be on the borderline ofconduction, i.e.,

transistor TR2 will have a very small collector current, so that thevoltage across resistor R4 will be near zero or very small.

When a signal is being received by antenna 12, the A.G.C. signal appliedto the base electrode of transistor TR2 will become less negative, andtransistor TR2 will be conducting. The voltage at terminal 26 then willbecome less negative, the'collector current of transistor TR1 willdecrease, and the gain of the RF. amplifier will decrease as required inorder to compensate any increase in the signal being received by antenna12. When transistor TR2 becomes conductive, the voltage at the collectorelectrode of transistor TR2 will change from essentially groundpotential to a negative value, since collector current will pass throughresistor R4. However, provided that the signal being received'by antenna12 has not increased to near the point of overload, and, assumingresistor R4 has been chosen properly, the change in voltage at thecollector electrode of transistor TR2 will be insurfficient to causediode D1 to conduct. Therefore, the variable impedance network shuntingtuned input circuit 10 and consisting of diode D1 and resistor R4 willremain effectively an open circuit.

Under near overload conditions the A.G.C. signal on line 23 will swingsufficiently less negative from its weak signal value to causetransistor TR2 conduct heavily. When transistor TR2 conducts heavily,the voltage at terminal 26 will become even less negative than before,causing transistor TR1 to become non-conductive. At the same time, theincreased collector current of transistor TR2 will cause its collectorelectrode to become sufficiently negative to cause diode D1 to conductcontinuously. As soon as diode D1 conducts continuously, there will be alow impedance path shunting tuned input circuit 10, and the tuned inputcircuit will be damped to such a degree that the RF. signal at terminal13 will be appreciably attenuated, so that even if this signal iscoupled from tuned input circuit 10 to tuned output circuit 11 by anymagnetic coupling between coil L1 and coil L2 or via stray capacitance,it will be unable to overload the mixer stage.

The value of resistor R4 determines the point at which diode D1 willconduct, and resistor R4 should be chosen so that diode D1 will notconduct until a signal that is nearly in the overload region is beingreceived. Under these circumstances, the selectivity of the antennacircuit will be preserved until diode D1 conducts continuously. It alsoshould be noted that the provision of such a variable impedance networkshunting tuned input circuit 10 permits the use of a larger antenna thanwould otherwise be possible, thereby increasing the sensitivity of thereceiver and the signal to noise ratio for weak signals.

It is important to note that with A.G.C. network embodying thisinvention, the swing in voltage at terminal 26 required to causetransistor TR1 to become non-conductive is very small. Thus withtransistor TR1 on the borderline of conduction, the voltage of terminal26 may be 11 volts (no signal), whereas, under strong signal conditions,a swing in voltage at terminal 26 of the order of 0.2 to 0.3 volt to10.8 or 10.7 volts will cause transistor TR1 to become non-conductive.In efiect, a current trading action takes place atterminal 26. Under nosignal conditions, there will be emitter current flowing in transistorTR1 but not in transistor TR2. Under conditions where the signal beingreceived is of weak or medium strength, there will be emitter currentflowing in both transistors, and, under strong signal conditions, therewill be no emitter current flowing in transistor TR1, but emittercurrent will flow in transistor TR2.

It should be noted that with the system of FIGURE 1, it is not necessaryto apply a negative (with respect to ground) potential to the anode ofdiode D1 to ensure that this diode is held off from continuousconduction except under near overload conditions, since a silicon diode,as

used here, requires approximtaely .6 volt to start conduction. Thus, theanode of diode D1 can be connected to ground, as shown.

Because it is only necessary to overcome about 0.2 volt of the 1 voltemitter voltage of tranistor TR1 before full A.G.C. action occurs, theA.G.C. power source (LF. amplifier 18) will be lightly loaded. Thesystem also provides two negative-going voltages (at terminal 26 and thecollector electrode of transistor TR2) to perform A.G.C. and overloadcontrol functions, these voltages commencing at the required D.C.values.

The operation of the network shown in FIGURE 2 is essentially the sameas the operation of the network of FIGURE 1, except that the A.G.C.signal is applied to the emitter electrode of transistor TR3 of LF.amplifier 18, rather than to the emitter electrode of transistor TR1, sothat, under near overload conditions, the gain of the LP, amplifier willbe materially reduced and tuned input circuit will be shunted by the lowimpedance network consisting of conducting diode D1 and resistor R4. IfA.G.C. is applied to transistor TR1, the R.F. amplifier transistor willbe non-conductive under near overload conditions.

Of course, PNP transistors can be substituted for NPN transistors TR1and TR2 by changing B" to B+. In this event, it also would be necessaryto change the polarity of diode D1, i.e., to connect its anode to thecollector electrode of transistor TR2 and its cathode to terminal 13.

It also should be noted that transistors TR1 and TR2 could be fieldeffect transistors. Therefore, where herein reference is made to atransistor having base, collector and emitter electrodes, thisterminology is intended to include a field effect transistor having agate, drain and source respectively.

Furthermore, it is not essential for the variable impedance network tobe connected across the whole of tuned input circuit 10. Thus, thevariable impedance network could be connected across only antenna coil12 or across coil L1 or a part thereof, the governing considerationbeing that the tuned input circuit must be damped when D1 conductscontinuously.

Strictly by way of example, the following components of the system ofFIGURE 1 may be as follows:

While preferred embodiments of this invention have been disclosedherein, those skilled in the art will appreciate that changes andmodifications may be made therein without departing from the spirit andscope of this invention as defined in the appended claims.

What I claim as my invention is:

1. In a signal receiving system of -a type comprising an R.F. signalamplifier having a tuned input circuit and an LP. signal amplifier, oneof said amplifiers including a first transistor having base, collectorand emitter electrodes; a source of an A.G.C. signal; and a networkinterconnecting said one signal amplifier and said source for supplyingsaid A.G.C. signal to said one signal amplifier to automatically controlthe gain thereof; the improvem'ent wherein said network includes anA.G.C signal amplifier, said A.G.C. signal amplifier including a secondtransistor having base, collector and emitter electrodes; means D.C.connecting said source and said base electrode of said second transistorfor supplying said A.G.C. signal to said base electrode of said secondtransistor; means D.C. connecting said emitter electrodes of said firstand second transistors together; a DC. power supply; and means includinga resistor providing a DC. path between said DC. power supply and bothof said emitter electrodes and wherein said signal Ieceiving systemincludes a variable impedance network connected across at least a partof said tuned input circuit and to said second transistor and having ahigh impedan-ce when said second transistor is just conducting and a lowimpedance when said second transistor is conducting heavily, wherebyunder near overload conditions said variable impedance network dampssaid tuned input circuit.

2. The invention according to claim 1 wherein said one signal amplifieris an R.F. amplifier.

3. The invention according to claim -1 wherein said one signal amplifieris an LP. amplifier.

4. The invention according to claim 1 wherein said variable impedancenetwork comprises a diode poled to conduct when said second transistoris conducting heavily and connected between said tuned input circuit andsaid collector electrode of said second transistor, and a secondresistor connected between said collector electrode of said secondtransistor and a terminal at a reference potential.

5. The invention according to claim 4 wherein said variable impedancenetwork is connected across said tuned input circuit.

6. The invention according to claim 4 wherein said diode is a silicondiode.

7. The invention according to claim 1 wherein said variable impedancenetwork is connected across said tuned input circuit.

References Cited UNITED STATES PATENTS 3,002,090 9/1961 Hirsch 325411XR3,264,564 8/1966 Guggi 325411 3,356,951 12/1967 Willard 325-411XR3,036,276 5/1962 Brown 325411 ROBERT L. GRIFFIN, Primary Examiner C. R.VON HELLENS, Assistant Examiner US. Cl. X.R. 325-680, 478

